Surge suppression devices are well known in the art for protecting sensitive electronic devices from electrical surges due to power line fluctuations and lightning, for example. In particular, electronic devices that receive RF signals from antennas or transmission lines (which are typically coaxial cable) are particularly susceptible to electrical surges, because a) transmission lines often carry electrical power signals as well as information signals; and b) transmission lines are typically suspended above the ground, attached to poles or other structures for long distances where they are susceptible to lightning strikes and power interruptions due to broken lines. Lightning strikes are known to reach potentials of 5 to 20 million volts with currents of thousands of amps and thus pose a significant threat to downstream electronic equipment.
Several types of surge suppressors have been proposed. Gas type surge suppressors contain gas that is ionized by the increase in voltage due to the electric surge and the ionized gas conducts the excessive electricity to ground. Metal Oxide Varistor (MOV) surge suppressors contain voltage sensitive semiconductors that shunt the excessive electricity to ground. Inductor-capacitor or L-C type surge suppressors typically include a capacitive element connected in series with the signal conductor, and an inductor coupled between the signal conductor and ground, typically through a housing that is connected to the outer conductor of the transmission line. The capacitance value of the capacitive element is selected so as to allow the desired RF signals to pass relatively unimpeded, but to block electrical surges which typically occur well below RF frequencies (e.g. between DC and 30 KHz in the case of lightning). In contrast, the value of the inductor is selected so as-to conduct the electrical surges to ground while blocking the RF signal. The combination of the capacitive element and inductor forms an L-C filter, which must be tuned to achieve the desired input impedance over the operating frequency range for low VSWR (Voltage Standing Wave Ratio) and insertion loss.
U.S. Publication 2004/0042149 discloses an L-C type surge suppressor for serial in-line connection with a coaxial cable to protect electronic equipment from electrical surges, particularly due to lightning. The surge suppressor of U.S. '149 includes an inner conductor comprised of two conductive portions shaped as plates, mechanically coupled together through a dielectric material to form a capacitor, an outer conductor electrically insulated from the inner conductor and an inductor coupling the inner conductor to the outer conductor. The capacitor and inductor values are selected so as to form an L-C filter properly tuned for the bandwidth of operation. The surge suppressor further has an input port shaped and configured as a coaxial connector and a protected output port also shaped and configured as a coaxial connector. Electrical surges that enter the input port are blocked by the capacitor and coupled by the inductor to the outer conductor and, thus, to ground.
For the purported ease of manufacture, the inductor of U.S. '149 is mechanically and-electrically coupled to the outer conductor by staking and is mechanically and electrically coupled to the inner conductor through a restorative force created by a bent portion of the inductor. As such, the inductor is coupled to the inner and outer conductors via solderless connections. These types of connections, however, cause the passive L-C components to act non-linearly, thus significantly reducing the current handling capability and degrading the passive intermodulation performance of the surge suppressor. Additionally, the solderless connections and physical configuration of the inductor make it susceptible to deformation by electromagnetic forces created by the high pulse currents associated with a lightning surge. Deformation of the inductor will change the frequency response characteristics and eventually lead to failure of the surge suppressor to properly conduct the electrical surge to ground, thereby damaging the device and possibly downstream electronic components.
U.S. Pat. No. 6,236,551 discloses a surge suppressor device similar to that of U.S. '149. However, the inductor of U.S. '551 is a spiral inductor. The spiral inductor is comprised of a high tensile strength material to inhibit the above mentioned deformation and to provide increased current carrying capability. However, the design and tuning processes for such spiral inductors are complicated and time consuming, requiring multiple design and manufacturing iterations and testing to achieve the desired input impedance for low VSWR and insertion loss.
What is needed, therefore, is a surge suppressor for a transmission line capable of handling large amounts of surge current with improved passive intermodulation performance that is relatively easy to design and cost effective to manufacture.